1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system. More particularly, the present invention relates to a method and apparatus for indexing physical channels in an OFDM wireless communication system.
2. Description of the Related Art
Active research is being conducted on the utilization of OFDM for a wireless communication system. OFDM is a special type of Multi-Carrier Modulation (MCM) in which an input serial symbol sequence is converted into parallel sequences and modulated to be mutually orthogonal multiple subcarriers, prior to transmission.
OFDM is more often used in digital transmission technologies. Examples of OFDM utilized in digital transmission technologies include Digital Audio Broadcasting (DAB), digital television, and Wireless Local Area Network (WLAN). Due to OFDM's robustness against multipath fading, OFDM provides an efficient platform for high-speed data transmission.
A major multiple access system based on OFDM is Orthogonal Frequency Division Multiple Access (OFDMA). In OFDMA, the frequency domain is divided into subchannels each having a plurality of subcarriers, the time domain is divided into a plurality of time slots, and the subchannels are allocated to different users. OFDMA, which allocates resources in the time-frequency domain, can accommodate numerous users while only using limited resources.
FIG. 1 illustrates an example of time-frequency resources in a conventional OFDM wireless communication system.
Referring to FIG. 1, the horizontal axis represents time and the vertical axis represents frequency. Since the OFDM system typically sends one modulation symbol (one Quadrature Phase Shift Keying (QPSK) or 16-ary Quadrature Amplitude Modulation (16 QAM) symbol) on one subcarrier 101, the subcarriers are basic resources. One rectangle representing one subcarrier in a specific OFDM symbol is time-frequency resources.
Generally, each OFDM symbol 102 includes a plurality of subcarriers. As shown in FIG. 1, all of the subcarriers are data subcarriers for delivering data, with there being no guard subcarriers. A basic packet transmission unit, called a Transmission Time Interval (TTI) 103, is formed with a set of OFDM symbols.
In FIG. 1, each small rectangle is called a time-frequency bin and a TTI 103 is comprised of a plurality of time-frequency bins. Physical channels are channels carrying different types of information like a paging channel, a Packet Data CHannel (PDCH), a Packet Data Control CHannel (PDCCH), and an uplink scheduling channel used in a typical mobile communication system.
Typically, one TTI 103 includes a plurality of physical channels. For instance, some time-frequency bins are used for the paging channel and others are used for a Common Control CHannel (CCCH) for providing system information during the TTI 103. Also, some time-frequency bins are allocated to the PDCH and others to the PDCCH for providing control information needed to demodulate the PDCH during the TTI 103. While not specified herein, other physical channels may be defined according to their purposes.
The physical channels require different time-frequency resources. Given 5,000 time-frequency bins for one TTI (i.e. 10 OFDM symbols for one TTI, each OFDM symbol including 500 data subcarriers), for example, resources can be allocated such that the paging channel has 100 subcarriers, the CCCH uses 500 subcarriers, 4000 subcarriers are used to deliver user data, and 400 subcarriers are used for sending PDCCHs.
In the above typical OFDM wireless communication system, resources are two-dimensionally configured in time and frequency and a plurality of physical channels require different amounts of resources. Therefore, the allocation of the time-frequency bins to the physical channels must be efficiently specified and also, the time-frequency bin allocations must be communicated between a transmitter and a receiver. If 5,000 subcarriers exist in one TTI, the transmitter must be able to notify the receiver that subcarriers #1 to #100 are for the paging channel and subcarriers #101 to #600 are allocated to a common channel. For this purpose, each physical channel can be identified using OFDM symbol indexes and subcarrier indexes. However, this method is inefficient because this method takes too much information to identify the subcarriers of each physical channel.
Specifically, when a plurality of PDCHs are multiplexed in a TTI, each PDCH is identified by a channel index. In correspondence with each channel index, OFDM symbols and subcarriers allocated to the channel of the channel index are preset between the transmitter and the receiver.
FIG. 2 illustrates an example of indexing a plurality of physical channels in the conventional OFDM wireless communication system.
Referring to FIG. 2, the horizontal axis represents time and the vertical axis represents frequency. Each OFDM symbol 202 includes a plurality of subcarriers and each TTI 203 is comprised of a plurality of physical channels. Each physical channel is equivalent to a Time-Frequency Diversity CHannel (TFDCH) 204, 205 or 206 in that one TFDCH is configured with a plurality of subcarriers scattered in time and frequency in a TTI 203. As illustrated in FIG. 2, subcarriers are allocated to each TFDCH for a specific TTI in a predetermined method and information about resources used for the TFDCH is defined by a corresponding channel index between the transmitter and the receiver.
The above conventional technology efficiently indicates subcarriers used for each physical channel in the case where all physical channels are of the same channel length (i.e. the same number of subcarriers are allocated to each physical channel during a TTI). However, when the physical channels have different channel lengths, it has limitations in indexing or indicating resources used for each physical channel.
Accordingly, there is a need for an improved method and apparatus for indexing physical channels in an OFDM wireless communication system for physical channels have different channel lengths.